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Stink bugs threaten Southeast corn crop

Stink bug damage in the Southeast has been most widely reported in cotton. However, these stinky pests have steadily become a threat to virtually every crop grown in the region, including corn.

Increased interest in grain crops in the upper Southeast in recent years may be one cause of the increase in stink bug damage in the past few years. However, the over-riding reason is the dramatic drop in the use of pyrethroid and organophosphate insecticides.

Stink bugs are still easily managed with either of these families of insecticides, which were rendered virtually obsolete by the inclusion of Bacillus thuriengensis traited cotton seed and most grain crops grown in the region.

While Bt crops played an economically critical role in managing other major insect pests in the region, combined with increased acreage in grain crops and in particular no-till grain crops, it appears to have produced a near perfect environment for stink bugs.

University of Kentucky Entomologist Ric Bessin says stink bug damage in corn was first recognized as a problem in Kentucky in 1985. Common occurrence, he says, is for a very small percentage of fields to be affected by stink bugs. However, damage has been recorded throughout the state.

Stink bugs feed on a wide variety of cultivated crops, weeds and native vegetation. Soybeans are a favorite host of stink bugs and increased acreage in recent years has provided a late-season host for this pest. Combined with increased popularity of no-till systems, the result has created a better environment for stink bugs to feed on and damage young corn plants.

As corn in the upper Southeast nears the growth stages critical to ear and kernel development, growers should be aware of the potential threat of stink bug damage.

Stink bugs can reduce yields when feeding kills small seedlings, resulting in stand reduction. Surviving plants are stunted and generally have reduced root mass. These plants may then be more susceptible to other stress-producing factors such as dry weather or attack by other insects.

Stunted plants may catch up in height with undamaged plants in 2 to 4 weeks, but research at the University of Kentucky indicates that yield from these plants may be reduced by 10 percent or more.

Stink bug damage is most severe in no-tillage fields. In this case, the damage can be found throughout the field, often with areas of more intense damage near wooded areas. Stink bug damage can be found in conventional fields, but the incidence of damaged plants is low and usually limited to the border rows.

Surveys in Kentucky and adjacent states have identified a soybean-wheat-corn cropping sequence as especially favorable for stink bug damage. A stink bug population can build up in soybeans during podfill. Wheat cover crops provide an attractive early spring host for the insects, and subsequently they feed on emerging corn. The stink bugs may over-winter in the wheat stubble, or they may leave the field for over-wintering sites and return in the spring.

Stink bug injury to corn is usually more common following a mild winter. Though the winter of 2010 wasn’t exactly mild in the upper Southeast, the record amounts of snowfall triggered by El Nino did not produce comparable cold weather records. The lingering effects of El Nino has produced a wet spring, which doesn’t bode well for stink bug feeding on corn.

Corn that has been planted into a wheat or rye cover crop is more susceptible to economic problems. Both nymphs and adults insert their needlelike mouthparts into the tender stems of corn plants, thereby introducing enzymes into the vascular system of young plants. These compounds can elicit phytotoxic symptoms and/or lead to growth abnormalities, such as "suckering" or profuse tillering.

Other symptoms on plants may include the presence of very small feeding holes surrounded by yellow or decaying plant tissue. Severely injured plants may become stunted, wilted, and subsequently die.

Stink bug injury limited to even one day of feeding has been reported to cause significant yield reductions. Fields most likely to support economic infestations of stink bugs include those fields with seed slots that have not been closed properly. This often occurs when fields have been planted too wet and/or the planter settings have not been adjusted to match field conditions.

The most dramatic symptom is tillering of damaged plants, which usually occurs approximately 10 days after stink bug damage occurs. A shoot begins to grow from the base of the plant and may become as large as the original corn plant. Stink bug-damaged corn plants often develop stunted and miss-shaped ears in place of the tassel. Visible damage often mirrors herbicide damage.

Unlike herbicide damage, stinkbugs will leave "styletes" in the plant tissue at the site of feeding. Laboratory staining of the leaves to detect the styletes can provide positive diagnosis of stinkbug feeding, according to Bessin.

Unlike many insect pests that cause problems in corn, brown stink bugs are relatively easy and inexpensive to control. Knowing when, or if, to treat is the big question.